APRIL FOOLS is somewhat of an April Fools' Day bait-and-switch video, made by SuperMarioGlitchy4.
SMG4 cuts the filming of Mario's video short as an April Fools' Day joke, then begins dancing along with random people in the background.
Mario is apparently filming an exercise video in the castle lobby: he opens the blooper asking the viewers if they are ready to Mario-cise. Just as he is about to begin the workout, however, SMG4 runs on-screen and punches Mario to the ground with a punch. Large text is edited in, reading "MISSION ACCOMPLISHED!"; at the same time, loud music and an explosion from out of the blue prompt several random characters to appear and start dancing.